The technical field of this invention is radiology and, in particular, bone absorptiometry by radiographic measurements.
The depletion of bone mineral content, typically referred to as osteoporosis, is a common consequence of a variety of diseases and natural aging processes. In addition to metabolic bone diseases and aging, bone minerals can be lost as the result of drugs, stress, dietary deficiencies, pregnancy or lactation. When skeletal bone mass drops below the level necessary to provide mechanical support, the depletion of bone mineral content becomes an important cause of morbidity, particularly in elderly patients.
Unfortunately, at present there are no reliable and inexpensive systems for gauging bone mineral content (BMC) with any high degree of precision, particularly during the early stages of osteoporosis or other mineral depletion disorders when dietary supplements and therapeutic agents may reverse the course of demineralization and prevent debilitating fractures or otherwise slow the progress of the disease.
Conventional methods for determining bone mineral content typically involve measurements of radiation absorption in the bone. U.S. Pat. No. 3,715,588 issued to Rose on Feb. 6, 1973, is illustrative of a prior art "bone scanner" in which a collimated X-ray beam is passed through a bone (e.g., the wrist) and detected by a radiation detector mechanically coupled to the X-ray source. The system scans back and forth across the bone to produce a complete measurement of the bone and surrounding tissue. Because of inherent differences in tissue and bone absorPtion, bone density (and, hence, mineral content) can be inferred from a logarithmic ratio of the intensity of radiation detected after transmission through the two media.
Other techniques for determining bone mineral content using stationary "area" scintillation cameras have also been proposed. See, for example, DePuey et al., "Bone Mineral Content Determined by Functional Imaging", Vol. 16, J. Nuclear Medicine, pp. 891-895 (1975), for a discussion of such scintillation camera-based systems. The area detectors similarly rely upon the inherent differences between tissue and bone absorption of radiation to compute bone density values.
Both linear scanning and area detection systems suffer from a lack of precision in measurement. In scanning systems, the motion of the radiation source and detector can result in image blurring, particularly if patient motion occurs. In area systems, the resolution of the detector (e.g., a scintillation medium) can be a limiting factor.
These limitations can be compounded by readout problems, particularly when a digital readout from an area detector is desired. Typically a radiation event will trigger a response over an entire region of the detector and analog signals generated by the numerous detector elements must be transferred by the detector (e.g., via delay lines) to a data acquisition and/or image. Conventional readout systems can be overwhelmed by the magnitude and/or near coincidence of radiation events.
There exists a need for readout systems for radiographic imaging, which would permit higher counting rates and thereby reduce processing bottlenecks, total measurement time and motion-related imaging errors. Moreover, there exists a need for readout systems which can digitize analog signals from detector elements and identify from a regional detector response the locus of the radiation event.